Currently, photosensitive lithographic printing plates (PS plates), etc., are being used as lithographic offset printing plates. These have either a negative photosensitive agent consisting mainly of acrylic monomers or prepolymers, or a positive photosensitive agent composed mainly of phenol resin and diazo compounds. However, these are all of low sensitivity, so that plate making is effected by contact exposure of a silver salt photographic film precursor on which the image has been pre-recorded. Over recent years, however, electronic editing systems have come into practical use: advances in computer image processing and large capacity data storage and data communications technologies have made it possible to handle all processes from entering text to correcting, editing, layout and binding by computer in an integrated fashion. Such systems are able to send their output to terminal plotters in distant locations instantaneously via high speed communications networks or satellite links. The degree of demand for electronic editing systems is particularly high in the field of newspaper printing, where speed is a requirement. In addition, with the development of ultralarge capacity recording media such as optical disks, it is considered that for fields in which printing plates are duplicated as they are required by storing originals in the form of baseplate films, it will become possible to store originals on such recording media in the form of digital data.
Known printing plate materials (printing plate precursors) in which electrophotography is made use of are, for example, zinc oxide/resin dispersion system offset printing plate materials as disclosed in, for example, JP-B-47-47610, JP-B-48-40002, JP-B-48-18325, JP-B-51-15766 and JP-B-51-25761 (the term "JP-B" as used herein refers to an "examined Japanese patent publication") and these are used after the formation of a toner image by electrophotographic methods, and after moistening with an oil-desensitizing solution (for example, an acidified water solution containing ferricyanide salts or ferrocyanide salts) to make the nonimage area oil-desensitive. Offset printing plates which have been processed in this manner have the capacity to withstand printing 5,000 to 10,000 sheets. However, they are not appropriate for printing, more than this and have a number of disadvantages: static electrical properties are poor and image quality deteriorates when a composition having an oil-desensitive property is employed. In addition, there is the disadvantage that harmful cyanide compounds are used as the oil-desensitizing solutions.
The resin printing plate having organic photoconductive materials disclosed in, for example, JP-B-37-17162, JP-B-38-7758, JP-B-45-39405, JP-B-52-2437 make use of an electrophotographic photoreceptor in which a photoconductive insulated layer, in which oxazole or oxadiazole compounds are bound by a styrene/maleic anhydride copolymer, is set on a sand-roughened aluminum plate; after a toner image is formed electrophotographically on this photoreceptor, a printing plate is made by removing the nonimage area with an alkaline organic solvent.
In relation to the above-mentioned method, a method has also been proposed for using an alkaline aqueous solution containing an organic solvent as the processing fluid for removing the nonimage area.
In the processing of the above-mentioned electrophotographic photoreceptor having a toner image on the photoconductive layer (the lithographic printing plate precursor), the above-mentioned processing fluid is applied to the surface of the photoconductive layer by spraying with a spray or by immersion, etc., or is spread over the surface with a brush roller, etc., and the nonimage area of the photoconductive layer is removed.
When carrying out this sort of processing in respect of a large number of electrophotographic photoreceptors using the same processing equipment, it is necessary to change or supplement the processing fluid because, as the processing progresses, the processing fluid deteriorates by the consumption of a certain component of the processing fluid and by the decrease of the pH of the fluid due to the involving of a CO.sub.2 gas to the fluid from air, insufficient elution has an adverse effect on the graphic quality of what is printed. Checking the extent to which this processing fluid has deteriorated and replenishing the processing fluid is troublesome. In addition, replenishing processing fluid after it has deteriorated leads to printing plates which are poor in parts.
For this reason, it is desirable to add processing fluid (replenisher) automatically: for example, adding the replenisher in accordance with processing time or processing parameters for the electrophotographic photoreceptor (for example, the number and length of photoreceptors introduced into the processing machine), has been considered. However, the area of the toner image which is formed on this type of photoreceptor differs with different photoreceptors, and consequently, the area of the nonimage area which is removed differs and therefore the degree of deterioration of the processing fluid differs with different photoreceptors, making it ultimately impossible to add fluid correctly.